Add the user satisfaction survey

This branch adds a bar for the user satisfaction survey to every page of GOV.UK. It is hidden by default, and appears to 1 in 6 users who visit the site. 

Once a user has taken or dismissed the survey, a cookie is set to remember this. When the user takes the survey, we append the current path of the page they are on into the survey url, so we can capture it with SurveyMonkey.

This branch is quite old, so there's still some things to iron out. @KushalP is going to take a first look to check over the cookie implementation.

Please don't merge until the associated work to document the cookie (and survey cookies) in frontend has been completed.